Lighting system of a microlithographic projection exposure system and method for operating such a lighting system

ABSTRACT

An illumination system of a microlithographic projection exposure apparatus includes a light source operated in a pulsed fashion and an array of optical elements which are digitally switchable between two switching positions. The array may be produced using MEMS technology.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of, and claims benefit under35 USC 120 to, U.S. application Ser. No. 15/236,725, filed Aug. 15,2016, which is a continuation of, and claims benefit under 35 USC 120to, international application PCT/EP2015/000273, filed Feb. 10, 2015,which claims benefit under 35 USC 119 of German Application No. 10 2014203 041.2, filed Feb. 19, 2014. The entire disclosures of theseapplications are incorporated by reference herein.

FIELD

The disclosure relates to an illumination system of a microlithographicprojection exposure apparatus, the illumination system including a lightsource operated in a pulsed fashion and an array—produced e.g. usingMEMS technology—of optical elements which are digitally switchablebetween two switching positions.

BACKGROUND

Integrated electrical circuits and other microstructured components areusually produced by virtue of a plurality of structured layers beingapplied onto a suitable substrate, which is usually a silicon wafer. Forthe purposes of structuring the layers, these are initially covered by aphotoresist (resist), which is sensitive to light from a specificwavelength range, e.g. light in the deep ultraviolet (DUV), vacuumultraviolet (VUV) or extreme ultraviolet (EUV) spectral range.Subsequently, the wafer coated thus is exposed in a projection exposureapparatus. Here, a pattern of diffractive structures, which is arrangedon a mask, is imaged onto the resist with the aid of a projection lens.Since the absolute value of the imaging scale generally is less than 1in this case, such projection lenses are sometimes also referred to asreduction lenses.

The wafer is subjected to an etching process after developing theresist, as a result of which the layer is structured in accordance withthe pattern on the mask. The resist which still remained is then removedfrom the remaining parts of the layer. This process is repeated so oftenuntil all layers are applied to the wafer.

The prior art has disclosed illumination systems which use mirror arraysin order to be able to variably illuminate the pupil plane of theillumination system. Examples thereof are found in EP 1 262 836 A1, US2006/0087634 A1, US 2010/0060873 A1, US 2010/0277708 A1, U.S. Pat. No.7,061,582 B2, WO 2005/026843 A2 and WO 2010/006687 A1. In general, theseare mirror arrays in this case, in which the mirrors can be tiltedcontinuously over a certain angle range.

WO 2012/100791 A1 has disclosed an illumination system whichadditionally includes a digitally switchable micromirror array. Thismicromirror array is imaged onto the light-entrance facets of an opticalintegrator with the aid of a lens. A similar, but differently drivenillumination system is known from the European patent application filedon Nov. 22, 2013 and having the application number EP 13194135.3,entitled “Illumination System of a Microlithographic Projection ExposureApparatus”, the content of which is incorporated by reference in thesubject matter of the present application.

EP 2 202 580 A1 discloses an illumination system including a micromirrorarray, but the latter is not digitally switchable between two switchingpositions, but rather is continuously tiltable over a relatively largetilting angle range. The micromirror array is used for setting theillumination setting and is synchronized with the light source such thata change of the illumination setting can take place between two lightpulses.

Time division multiplexing operation of two light sources is known fromU.S. Pat. No. 5,880,817 and US 2007/0181834 A1.

SUMMARY OF THE DISCLOSURE

The present disclosure seeks to provide an illumination system whichcontains an array of optical elements which are digitally switchablebetween two switching positions, and which has particularly stableoptical properties.

In one aspect, the disclosure provides an illumination system of amicrolithographic projection exposure apparatus including a light sourcedesigned for generating a sequence of light pulses. The illuminationsystem furthermore includes an array of optical elements which aredigitally switchable between two switching positions. A control deviceof the illumination system is designed to drive the optical elementssuch that they change their switching position only between twosuccessive light pulses and maintain their switching position during thelight pulses.

The disclosure is based on the insight that the optical elements of thearray should be synchronized with the light source such that changes inthe switching position take place exclusively in the time intervalsbetween successive light pulses. Since the light pulses are generallyshort and the switching process likewise extends over a certain timeinterval, it is difficult to carry out the switching process within alight pulse at a predefined point in time. This would be theprerequisite for obtaining defined relations during the illumination ofthe mask.

The disclosure takes a different path, by in principle avoiding suchswitching processes during a light pulse. As a result, the setting ofthe optical elements to which a specific light pulse is to be assignedis always defined unambiguously.

The control device can be designed to drive the optical elements suchthat the switching position of at least one of the optical elements isidentical during two or more successive light pulses. If the number oflight pulses during which the switching position is identical, duringthe illumination of a mask, is different in the case of differentoptical elements, it is possible to generate different brightness levelsin a manner integrated over time on a target surface illuminated by thearray. In this case, the number of light pulses can be made dependente.g. on measurements or simulations of the intensity on the targetsurface or in a surface that is optically conjugate with respectthereto.

In one exemplary embodiment, the optical elements move between theswitching positions. The control device is designed to drive the opticalelements such that the switching position changes 2·n, n=1, 2, 3, . . ., times between the two or more successive light pulses. By changing theswitching position, the optical elements move, which fosters themovement of the air (or some other gas) surrounding the optical elementsand thus the cooling of the optical elements.

The disclosure is not restricted to arrays including micromirrors, sincesimilar problems can also occur in other arrays including digitallyswitchable optical elements. In this regard, the optical elements canbe, for example, tiltable wedge prisms or liquid crystal cells such asare known from LCDs.

A lens can be arranged between the array and a target surface, the lensimaging the array onto the target surface. The target surface can be, inparticular, the light entrance facets of an optical integrator thatgenerates a multiplicity of secondary light sources in a pupil plane ofthe illumination system.

In general, the sequence of the light pulses will be periodic. However,the disclosure is also applicable to such light sources in which thelight pulses are not emitted periodically or are not emitted strictlyperiodically.

The light source can be, in particular, a laser designed for generatingprojection light and a center wavelength of between 150 nm and 250 nm.In principle, however, the disclosure is also usable for shorter centerwavelengths, for instance for center wavelengths in the EUV spectralrange.

In one exemplary embodiment, the illumination system includes a furtherlight source, which is designed for generating further light pulses. Inthis case, the further light pulses are generated in a manner offsettemporally with respect to the light pulses of the aforementioned lightsource. The array is designed to couple, in a first switching positionof the optical elements, light pulses of the aforementioned light sourceand, in a second switching position of the optical elements, lightpulses of the further light source into a common beam path of theillumination system. In this way, two light sources can be coupled intothe beam path without the input-side etendue of the illumination systemincreasing as a result. In comparison with conventional switchablecoupling-in elements such as tilting mirrors or rotating prisms, anarray of digitally switchable optical elements has the advantage that itinvolves very little structural space and has very short switching timeson account of the small masses to be moved. In this way, it is thuspossible to double the effective pulse rate of the illumination systemand hence the quantity of light available for the exposure of thelight-sensitive layer.

The disclosure furthermore relates to a method for operating anillumination system of a microlithographic projection exposureapparatus, including the following steps:

-   a) providing an array of optical elements which are digitally    switchable between two switching positions;-   b) generating a sequence of light pulses;-   c) changing at least twice the switching position of the optical    elements between two successive light pulses.

The optical elements preferably maintain their switching position duringthe light pulses. The array can be imaged onto a target surface with theaid of a lens.

In one exemplary embodiment, the array couples, in a first switchingposition of the optical elements, light pulses of the aforementionedlight source and, in a second switching position of the opticalelements, light pulses of a further light source into a common beampath.

The disclosure additionally relates to an illumination system of amicrolithographic projection exposure apparatus including a first lightsource designed for generating a sequence of first light pulses, andincluding a second light source designed for generating a sequence ofsecond light pulses, which are emitted in a manner offset temporallywith respect to the first light pulses. A control device is designed todrive an array of optical elements which are digitally switchablebetween two switching positions such that, in a first switching positionof the optical elements, exclusively light pulses of the aforementionedlight source and, in a second switching position of the opticalelements, exclusively light pulses of the further light source arecoupled into a common beam path of the illumination system.

The disclosure furthermore relates to a method for operating anillumination system of a microlithographic projection exposure apparatusincluding the following steps:

-   a) generating a sequence of first light pulses by means of a first    light source;-   b) generating a sequence of second light pulses by means of a second    light source, which are offset temporally with respect to the first    light pulses;-   c) driving an array of optical elements which are digitally    switchable between two switching positions in such a way that, in a    first switching position of the optical elements, exclusively light    pulses of the aforementioned light source and, in a second switching    position of the optical elements, exclusively light pulses of the    further light source are coupled into a common beam path of the    illumination system.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the disclosure will become apparentfrom the following description of exemplary embodiments with referenceto the drawings, in which:

FIG. 1 shows a much simplified perspective illustration of amicrolithographic projection exposure apparatus;

FIG. 2 shows parts of an illumination system according to the disclosurein a schematic perspective illustration;

FIG. 3 shows a graph in which the angular position of a micromirror isplotted as a function of the light pulses over time;

FIG. 4 shows parts of an illumination system according to the disclosurein accordance with another exemplary embodiment in an illustration basedon FIG. 2;

FIG. 5 shows a graph corresponding to FIG. 3 for the exemplaryembodiment shown in FIG. 4.

DESCRIPTION OF PREFERRED EXEMPLARY EMBODIMENTS

FIG. 1 shows a projection exposure apparatus 10 in a very schematicperspective illustration, the projection exposure apparatus beingsuitable for the lithographic production of microstructured components.The projection exposure apparatus 10 contains an illumination system 12including a light source LS, designed for generating projection lighthaving a center wavelength of 193 nm. The illumination system 12 directsthe projection light generated by the light source LS onto a mask 14 andilluminates there a narrow illumination field 16, which is rectangularin the exemplary embodiment illustrated. Other illumination field forms,e.g. ring segments, likewise come into consideration.

Structures 18 on the mask 14 lying within the illumination field 16 areimaged on a light-sensitive layer 22 with the aid of a projection lens20, which contains a plurality of lens elements L1 to L4. Thelight-sensitive layer 22, which may be e.g. a resist, is applied to awafer 24 or another suitable substrate and is situated in the imageplane of the projection lens 20. Since the projection lens 20 generallyhas an imaging scale |β|<1, the structures 18 lying within theillumination field 16 are imaged with reduced size on a projection field18′.

In the depicted projection exposure apparatus 10, the mask 14 and thewafer 24 are displaced along a direction denoted by Y during theprojection. The ratio of the displacement speeds in this case equals theimaging scale β of the projection lens 20. If the projection lens 20inverts the image (i.e. β<0), the displacement movements of the mask 14and of the wafer 24 extend counter to one another, as indicated in FIG.1 by arrows A1 and A2. In this manner, the illumination field 16 isguided in a scanning movement over the mask 14 such that even relativelylarge structured regions can be projected contiguously on thelight-sensitive layer 22.

FIG. 2 shows parts of the illumination system 12 according to thedisclosure schematically in a perspective illustration. The illuminationsystem 12 includes a carrier 26 for a micromirror array 28, onto whichthe light source LS directs projection light 30 directly or via furtheroptical elements (not illustrated). The micromirror array 28, which canbe realized as a DMD (digital mirror device), contains a regulararrangement of micromirrors 32 which are digitally switchable in eachcase between two switching positions. For this purpose, the micromirrorarray 28 is connected to a control device 34 via a signal connectionindicated in a dashed manner. Projection light 30 incident on themicromirror array 28, after deflection via a plane folding mirror 36, isdirected via a lens 38 onto a target surface 40, which may be forexample the surface of an optical integrator. In this case, the lens 38has the effect that the micromirror array 28 is imaged onto the targetsurface 40. In this way, the target surface 40 can be illuminatedvariably with the aid of the micromirror array 28.

FIG. 3 shows a graph in which the intensity of the projection light 30generated by the light source LS (right-hand ordinate) and the angularposition of one of the micromirrors 32 (left-hand ordinate) are plottedagainst time t. In this exemplary embodiment, it is assumed that thelight source LS generates light pulses 4-21 to 42-3 having the durationΔt and having a period T, wherein Δt/T«1. Consequently, a relativelylong time in which no projection light passes through the illuminationsystem 12 elapses between two successive light pulses 42. The pulsefrequency of the light source LS is typically of the order of magnitudeof a few kHz.

In the graph in FIG. 3 it can be discerned that the relatively long timeinterval between successive light pulses 42 is used to switch therelevant micromirror 32 multiply between its two switching positions,angles α_(on) and α_(off) respectively corresponding to the switchingpositions. During the light pulses 42, however, the relevant micromirror32 is always situated in a defined switching position, namely—during thefirst two light pulses 42-1 and 42-2—in a switching position in whichlight is directed onto the target surface 40 (α=α_(on)) and—during thethird light pulse 42-3—in a second switching position, in which no lightis directed onto the target surface 40 (α=α_(off)).

Multiply changing the switching position between successive light pulses42-1 to 42-3 fosters the movement of the air (or some other gas)surrounding the micromirrors 32 and thus the cooling of the micromirrors32. There is generally a desire for cooling because an (albeit a small)part of the high-energy projection light 30 incident on the micromirrors32 is absorbed by the reflective coating of the micromirrors 32 andconverted into heat. Cooling across micromirrors 32 solely via thecarrier 26 may therefore not suffice for the cooling. Cooling byconvection is particularly effective if the surrounding air is moved bythe micromirrors 32 changing their switching position.

FIG. 4 shows, in an illustration based on FIG. 2, another exemplaryembodiment wherein the micromirror array 28 is used to interlace firstlight pulses, which are generated by a first light source LS1 and secondlight pulses, which are generated by a second light source LS2, in themanner of time division multiplexing such that downstream of themicromirror array 28 the projection light 30 with double the pulsefrequency is directed onto the downstream optical elements of theillumination system 12.

For this purpose, the control unit 34 drives the micromirror array 28such that, in a first switching position, the micromirrors 32 coupleexclusively the first light pulses of the first light source LS1 and, ina second switching position of the micromirrors 32, couple exclusivelythe second light pulses of the second light source LS2 into a commonbeam path of the illumination system.

At the folding mirror 36, therefore, the first and second light pulsesare incident exactly from the same direction, but with a pulse frequencythat is doubled in comparison with the pulse frequency of eachindividual light source LS1, LS2.

This is illustrated by the graph shown in FIG. 5. The periodic sequencesof the first and second light pulses 42-1, 42-2 and 52-1, 52-2 areidentified by different etchings. The interlacing of the first lightpulses emitted with the period T1 and the second light pulses emittedwith the period T2=T1 yields a sequence of light pulses having aneffective period T_(eff)=T1/2=T2/2. The control device 34 drives therelevant micromirror 32 such that it is situated in its first switchingposition, which corresponds to the angle a₁, during the first lightpulses 42-1, 42-2, 42-3. During the second light pulses 52-1, 52-2, themicromirror is situated in its second switching position, whichcorresponds to the tilting angle a₂.

In this exemplary embodiment, too, the micromirror 32 is switchedbetween its two switching positions multiply between two successivelight pulses 42-1, 42-2, 42-3, 52-1, 52-2 originating from differentlight sources LS1, LS2, in order to improve the cooling by thesurrounding air.

What is claimed is:
 1. An illumination system, comprising: a first lightsource configured to generate pulses of light; a second light sourceconfigured to generate further pulses of light that are offsettemporally with respect to the pulses of light generated by the firstlight source; an array of optical elements which are digitallyswitchable between first and second switching positions; and a controldevice configured to drive the optical elements so that during use ofthe illumination system the switching position of the optical elementsis unchanged while any of the first light source and the second lightsource generates a light pulse, wherein: in the first switching positionof the optical elements, the array couples light pulses generated by thefirst light source into a common beam path of the illumination system;and in the second switching position of the optical elements, the arraycouples light pulses generated by the second light source into a commonbeam path of the illumination system; and the illumination system is amicrolithography illumination system.
 2. The illumination system ofclaim 1, wherein the control device is configured to drive the opticalelements so that during use of the illumination system the switchingposition of at least one optical element is identical during two or moresuccessive pulses of light generated by the first light source.
 3. Theillumination system of claim 2, wherein: the control device isconfigured to drive the optical elements so that during use of theoptical system the switching position changes 2·n times between the twoor more successive pulses of light generated by the first light source;and n is an integer having a value of at least one.
 4. The illuminationsystem of claim 1, wherein the target surface is a surface of an opticalintegrator.
 5. The illumination system of claim 4, wherein theillumination system is configured to illuminate a mask during use of theillumination system.
 6. The illumination system of claim 5, wherein thecontrol device is configured to drive the optical elements so thatduring use of the optical system the switching position changes 2·ntimes between the two or more successive pulses of light generated bythe first light source, and n is an integer having a value of at leastone.
 7. The illumination system of claim 4, wherein the control deviceis configured to drive the optical elements so that during use of theoptical system the switching position changes 2·n times between the twoor more successive pulses of light generated by the first light source,and n is an integer having a value of at least one.
 8. The illuminationsystem of claim 1, further comprising a lens in a path of the lightpulses generated by the first light source between the array and atarget surface, wherein the lens is configured to image the array ontothe target surface during use of the illumination system.
 9. Theillumination system of claim 2, further comprising a lens in a path ofthe light pulses generated by the first light source between the arrayand a target surface, wherein the lens is configured to image the arrayonto the target surface during use of the illumination system.
 10. Theillumination system of claim 3, further comprising a lens in a path ofthe light pulses generated by the first light source between the arrayand a target surface, wherein the lens is configured to image the arrayonto the target surface during use of the illumination system.
 11. Theillumination system of claim 4, further comprising a lens in a path ofthe light pulses generated by the first light source between the arrayand a target surface, wherein the lens is configured to image the arrayonto the target surface during use of the illumination system.
 12. Theillumination system of claim 5, further comprising a lens in a path ofthe light pulses generated by the first light source between the arrayand a target surface, wherein the lens is configured to image the arrayonto the target surface during use of the illumination system.
 13. Theillumination system of claim 6, further comprising a lens in a path ofthe light pulses generated by the first light source between the arrayand a target surface, wherein the lens is configured to image the arrayonto the target surface during use of the illumination system.
 14. Theillumination system of claim 7, further comprising a lens in a path ofthe light pulses generated by the first light source between the arrayand a target surface, wherein the lens is configured to image the arrayonto the target surface during use of the illumination system.
 15. Theillumination system of claim 1, wherein the illumination system isconfigured to illuminate a mask during use of the illumination system.16. The illumination system of claim 2, wherein the illumination systemis configured to illuminate a mask during use of the illuminationsystem.
 17. The illumination system of claim 3, wherein the illuminationsystem is configured to illuminate a mask during use of the illuminationsystem.
 18. The illumination system of claim 5, wherein the illuminationsystem is configured to illuminate a mask during use of the illuminationsystem.
 19. An apparatus, comprising: an illumination system accordingto claim 1; and a projection lens, wherein the apparatus is amicrolithographic projection exposure apparatus.
 20. A method ofoperating a microlithographic projection exposure apparatus comprisingan illumination system and a projection lens, the method comprising:using the illumination system to illuminate a pattern of a mask; andusing the projection lens to image at least some of the illuminatedpattern of the mask onto a light sensitive material, wherein theillumination system is an illumination system according to claim 1.